1. Field of the Invention
The present invention relates to a depression-type switch which generates a tactile "click" feel when it is turned on by depressing a key top or the like and which is designed to provide an overstroke, making it possible to further push in the key top or the like even after the switch has been turned on.
2. Description of the Related Art
In such a depression-type switch, a tactile click feel is transmitted to the finger of the operator when the key top is pushed in by a predetermined stroke, so that the operator can clearly feel with his finger that the switch has been turned on. Further, the stroke is not at an end immediately after the turning on of the switch but some more stroke (overstroke) is provided, so that an uncomfortable impact is not easily transmitted to the finger of the operator. Thus, a satisfactory operating feel can be expected. Further, the structure of the switch is relatively simple. Thus, nowadays, the switch is widely used in various fields.
FIG. 8 is a sectional view showing a conventional depression-type switch of this sort. As shown in the drawing, which schematically shows the construction of the switch, the depression-type switch, which is fitted in an opening 20a of a frame 20, includes a key top 11, a rubber spring 12, a coil spring 13 which is provided between the key top 11 and the rubber spring 12, and a membrane switch 14 which is provided underneath the rubber spring 12. The wall portion 12a of the rubber spring 12, which has a bowl-shaped configuration, is deformed (flips over) when pushed in from above by a predetermined amount, thereby generating a tactile click feel. The membrane switch 14 includes a flexible upper sheet 15, an upper electrode 16, which is provided on the lower side of the upper sheet 15, a lower sheet 17, lower electrodes 18 which are provided on the upper side of the lower sheet 17, and a spacer 19 which is provided between the sheets 15 and 17 and which has an opening 19a, the upper and lower electrodes 16 and 18 being capable of being brought into and out of contact with each other within the opening 19a.
In this construction, as the operator pushes in the upper surface of the key top 11 downwardly with the finger, the coil spring 13 is compressed by degrees, with the result that it increases in bounce. When the bounce of the coil spring 13 has reached a certain level, the rubber spring 12 flips over. When the rubber spring 12 flips over, the depressing protrusion 12b thereof pushes in the upper sheet 15 of the membrane switch 14 to thereby deflect it downwards, so that the upper electrode 16 is brought into contact with the lower electrodes 18, whereby the switch is turned on. Further, when the rubber spring 12 flips over, the bounce applied to the key top 11 through the coil spring 13 abruptly decreases, with the result that a tactile click feel is transmitted to the finger of the operator. Even after the rubber spring 12 has thus flipped over, the coil spring 13 can be compressed, so that the operator can further push in the key top by some amount while feeling the bounce increasing by degrees, whereby a satisfactory overstroke can be obtained. In this condition, when the push-in force of the finger applied to the key top 11 is cancelled, the rubber spring 12 is restored to its original bowl-shaped configuration, so that the deflection of the upper sheet 15 is cancelled. Thus, the upper electrode 16 is separated from the lower electrodes 18, thereby restoring the switch to the OFF state. At the same time, the key top 11 is pushed upward to the original height by the restoring force of the coil spring 13 and the rubber spring 12.
A specific example of such a depression-type switch is disclosed in Japanese Patent Laid-Open No. 60-50825, etc.
In the conventional depression-type switch described above, the rubber spring 12 having an appropriate height is mounted on a switch element, such as the membrane switch 14. Further, the coil spring 13 has to be mounted between the rubber spring 12 and the key top 11 so as to be capable of vertically expanding and contracting. Thus, it is rather difficult to form a thin switch whose general height is small. Further, in such a conventional switch, when an end depressing operation is performed by the operator, that is, when a peripheral portion of the upper surface of the key top 11 is depressed, the coil spring 13, which is inclined by such operation, is liable to be compressed unevenly, so that there is a concern that the switch will not operate in the normal fashion.